1. Field of the Invention
The present invention relates to a dielectric resonator. More specifically, the present invention relates to an improvement in the temperature characteristic of the resonance frequency in a dielectric resonator utilizing the TM.sub.010 mode or a modified mode thereof of an electromagnetic wave.
2. Description of the Prior Art
FIGS. 1 and 2 are views showing one example of a conventional dielectric resonator using the TM.sub.010 mode which constitutes the background of the present invention. More specifically, FIG. 1 is a longitudinal sectional view of the resonator and FIG. 2 shows a transverse sectional view of the resonator taken along the line II--II in FIG. 1. Referring to FIGS. 1 and 2, a dielectric resonator 1 comprises a case 2 formed wholly of metal, and a cylindrical dielectric member 4 of the length L disposed in a concavity 3, circular in section, defined in the case 2. An electromagnetic field distribution of the TM.sub.010 mode is shown therein, in which the solid line arrow 5 shows an electric line of force and a dotted line arrow 6 shows a magnetic line of force.
As shown in FIGS. 1 and 2, the TM.sub.010 mode is a mode in which the electric field is mostly concentrated inside the dielectric cylinder 4 and hence this mode enables miniaturization of the resonator 1. In such a case, the resonator 1 is effective for the TM.sub.010 mode and is less effective for the other modes. In this mode the resonance frequency f.sub.0 =C/.lambda..sub.0, (where C is a light velocity and .lambda..sub.0 is the resonance wave length) has no relation to the length of the resonator (the length of the cylindrical dielectric member) L. Accordingly, a dielectric resonator can be implemented in a smaller size.
Thus, a dielectric resonator using the TM.sub.010 mode or a modified mode thereof includes various advantages and hence can be advantageously utiized as a filter or an oscillating element.
However, a conventional TM.sub.010 mode dielectric resonator has the disadvantage that the temperature characteristic of a resonance frequency is not good. More specifically, assuming that the temperature characteristic of the resonance frequency is .eta..sub.f, then the following formula is obtained: EQU .eta..sub.f =-C.eta..sub..epsilon. -A.alpha..sub.1 -B.alpha..sub.2 ( 1)
where
.eta..sub..epsilon. : the temperature characteristic of the dielectric constant PA1 .alpha..sub.1 : the coefficient of linear expansion of the dielectric material PA1 .alpha..sub.2 : the coefficient of linear expansion of a metallic case PA1 A, B, C: constants
In other words, the temperature characteristic .eta..sub.f of the resonance frequency is related to the respective coefficients of linear expansion .alpha..sub.1 and .alpha..sub.2 of the dielectric material and the metallic case as well as the temperature characteristic .eta..sub..epsilon. of the dielectric constant. In order to make better the temperature characteristic .eta..sub.f of the resonance frequency, it is necessary to properly control the coefficients of linear expansion .alpha..sub.1 and .alpha..sub.2 of dielectric material and the metallic case as well as to select the temperature characteristic .eta..sub..epsilon. of the dielectric constant determined by the dielectric material. However, it is difficult to properly control simultaneously the coefficients of linear expansion .alpha..sub.1 and .alpha..sub.2 of the dielectric material and the metallic case in the light of the properties thereof. As a result, the temperature characteristic .eta..sub.f of the resonance frequency is poor.
Viewed from another angle, this means that a conventional resonator including a cylindrical dielectric material 4 disposed in a metallic case 2 exhibits a change in the small gap in the coupling region between the end surface 4a of the cylindrical dielectric material 4 and the facing surface 7 of the metallic case as a result of a change of the temperature around the resonator 1. This change results from a difference between the respective coefficients of linear expansion of the dielectric material 4 and the metallic case 2. The above described change in the gap occurring in the above described coupling region gives rise to a change in currents that flow in the devce, resulting in a change in the effective dielectric constant. This results in a change in the capacitance C which is one of the factors determining the resonance frequency f.sub.0 (f.sub.0 =1/2.pi..sqroot.LC). Accordingly, the conventional resonator has the disadvantage that a change in the resonance frequency occurs due to the temperature because difference between the coefficients of linear expansion of the case 2 and the dielectric material 4.
One example of an approach for eliminating the above described shortcomings is described in Japanese Laid Open Patent No. 119650/1978, laid open Mar. 29, 1977 and entitled "Very Small Sized Bandpass Filter Using E.sub.010 Mode of Dielectric Resonator". The above referenced Japanese Laid Open Patent is directed to a bandpass filter having an improved temperature characteristic using a resonator of the E.sub.010 (=TM.sub.010) mode, in view of the fact that a bandpass filter using H.sub.0.delta. mode has a worse spurious characteristic. More specifically, the resonator disclosed in the above referenced Japanese Laid Open Patent is adapted such that an aperture is formed on each end surface of a concavity in a metallic cylinder and both ends of the dielectric cylinder are extended into the end surfaces of the cylindrical concavity. As a result little influence is exerted upon the resonance frequency by expansion and contraction of the ends of the dielectric cylinder due to a change in the temperature.
Another conventional approach for improving the temperature characteristic of a dielectric resonator will be described in the following.
FIG. 3 is a longitudinal sectional view of another example of a conventional dielectric resonator.
Referring to FIG. 3, a dielectric resonator 10 comprises a conductive case 2 formed wholly of metal and defining a cylindrical concavity 3, and a cylindrical dielectric material 4 disposed concentrically at the center of the cylindrical concavity 3. Although the conductive case 2 is rigidly formed as a whole so as not to be readily deformed, only a bottom plate 2a of the case 2 is made to be as thin as 0.6 to 0.8 mm so as to be bent when the same is pressed with a finger.
An auxiliary case 11 is coupled to the bottom of the conductive case 2 by means of a coupling member 12, for example. A pressing member 13 and a dished spring 14 are disposed in the auxiliary case 11. The pressing member 13 is pressed toward the bottom plate 2a of the conductive case 2 by means of the dished spring 14. As a result, the bottom plate 2a is normally pressed upward by the pressing member 13, i.e. toward the bottom end of the cylindrical dielectric material 4 so as to be in contact with the lower end surface of the dielectric material 4. This contact is not changed by a change in the ambient temperature.
More specifically, if and when the ambient temperature of the resonator 10 changes, expansion and contraction of the conductive case 2 are larger than those of the dielectric material 4 due to a difference between the coefficients of linear expansion of the conductive case 2 and the cylindrical dielectric material 4 (generally the coefficient of linear expansion .alpha..sub.1 of a conductor is larger than the coefficient of linear expansion .alpha..sub.2 of a dielectric material), so that an increase in the temperature causes the bottom plate 2a of the conductive case to expand in the direction away from the bottom end surface of the cylindrical dielectric material 4, as shown by the solid line in FIG. 4, which shows a partial view of a portion encircled with the line IV in FIG. 3. However, since the bottom plate 2a is pressed toward the lower end surface of the dielectric material 4 by means of the pressing member 13 and the bottom plate 2a has elasticity, at least a portion of the bottom plate 2a pressed by the pressing member 13 is kept in close contact with the lower end surface of the cylindrical dielectric material 4.
Meanwhile, although the dielectric resonator 10 shown in FIGS. 3 and 4 was adapted to have an increased area of the portion where the bottom plate 2a is pressed by the pressing member 13, it is needless to say that the pressing member 13 is not necessarily an indispensable member and alternatively the resonator may be adapted such that the bottom plate 2a is directly pressed by the dished spring 14.
Preferably a contacting portion 13a of the pressing member 13 contacting the bottom plate 2a is selected to be at least of the same size or larger than the end surface of the cylindrical dielectric material 4, because this ensures that the bottom plate 2a is in close contact with the whole end surface of the dielectric material 4.
Since the dielectric resonator 10 shown in FIGS. 3 and 4 employed a dished spring 14 for the purpose of pressing the bottom plate 2a toward the end surface of the cylindrical dielectric material 4, the resonator is advantageous because a dished spring is compact, thin and stable. This permits the auxiliary case 11 to be accordingly compact. Alternatively, the bottom plate 2a may be pressed by a leaf spring, for example.
An aperture 15 is formed at the center of the dished spring 14 so that a protruding portion 13b of the pressing member 13 may be fitted thereinto. Such structure facilitates positioning of the pressing member 13.
Although the auxiliary case 11 was mounted to the conductive case 2 by means of the coupling member 12, alternatively the auxiliary case 11 may be shaped to enclose the whole of the conductive case 2.
Now referring to FIGS. 5 to 7, another example of a conventional resonator constituting the background of the present invention will be described in the following.
FIG. 5 is a longitudinal sectional view of this example of a conventional dielectric resonator, FIGS. 6A and 6B are partial views of a portion encircled with the line VI in FIG. 5, and FIG. 7 is a plan view of the resonator shown in FIG. 5. Referring to FIGS. 5 to 7, a dielectric resonator 20 comprises a conductive case 2 and a cylindrical dielectric material 4, as is the same as shown in FIGS. 3 and 4. The dielectric resonator 20 shown in FIG. 5 is characterized in that a groove 21 is formed on the outer surface of an upper plate 2b of the conductive case 2 contacting the upper end surface of the cylindrical dielectric material 4. The groove 21 is at a position corresponding to the periphery of the end surface of the dielectric material 4. Another groove 22 is also formed in the vicinity of the lower end portion of the side plate 2c of the conductive case 2. The groove 21 formed on the upper plate 2b may be of a circle of the same diameter as that of the section of the dielectric material 4 but may also be larger than that. The sectional shape of the grooves 21 and 22 need not be necessarily of a letter V in section and may of an arbitrary shape such as of a rectangle in section across its depth.
Since the above described grooves 21 and 22 are formed on the conductive case 2 in the above described manner, the conductive case 2 can be elastically bent only at these grooves 21 and 22.
Now assume that a force is applied in the direction of the arrow 23 shown in FIG. 5, i.e. in the direction of bringing the conductive case 2 in close contact with the end surface of the dielectric material 4. The upper plate 2b of the conductive case 2 is bent outward at the groove 21, as shown in FIG. 6A, when the ambient temperature surrounding the resonator 20 is low, because of a difference between the coefficients of linear expansion of the dielectric material and the metal. Conversely, if and when the ambient temperature is high, the metal expands more and the upper plate 2b is bent inward at the groove 21, as shown in FIG. 6B. In either event, i.e. irrespective of a change in the ambient temperature, the central portion surrounded by the groove 21 of the upper plate 2b is kept in contact with the end surface of the dielectric material 4, so that no gap is caused between the end surface of the dielectric material 4 and the conductive case 2. Meanwhile, it is to be pointed out that in FIGS. 6A and 6B deformation of the upper plate 2b has been shown in an exaggerated manner for purpose of illustration.
The groove 22 (FIG. 5) formed on the side plate 2c allow the side plate 2c to be bent inwardly about the groove 22 in accordance with the bending of the upper plate 2b. As a result, any distortion of the case 2 due to the bending of the upper plate 2b is absorbed by the side plate 2c, whereby no force is exerted upon the bottom plate 2a. In other words, the bottom plate 2a is normally kept flat as a whole. As a result, in applying such resonator 20 as a filter, for example, such a connector 24 as shown by the dotted line can be stably fixed to the bottom plate 2a.
A groove may be formed at the position of bottom plate 2a symmetrical to that of the upper plate 2b in place of the groove 22 formed on the side plate 2c.
Referring to the example described in the foregoing, the force in the direction of the arrow 23 (FIG. 5) to be applied to the conductive case 2 may be applied externally by means of a spring force. Alternately, by selecting the height of the cylindrical dielectric material 4 to be slightly larger than the height of the metallic case 2, a force can be applied normally in the direction of the arrow 23 as a function of the elasticity of the case 2 itself.
As a result of the above described structure, the temperature coefficient .eta..sub.of f the resonance frequency of the resonator has been measured for the conventional example (FIGS. 1 and 2) in which the conductive case is not changed and for the resonator of the other conventional examples shown in FIGS. 3 to 7. The results revealed that the temperature coefficient of the resonant frequency was greatly improved from 150 ppm/.degree.C. for the conventional resonators to approximately 10 to 20 ppm/.degree.C. for the last described example.
Although the above described dielectric resonators shown in FIGS. 4 to 7 employ countermeasures against a change in the resonance frequency due to thermal expansion, they still involve a problem of preventing a flow of a real current through the conductive case.
FIG. 8 is a view showing a flow of a real current through a conductive case of a dielectric resonator and FIG. 9 is a perspective view of a conductive case. As seen from FIG. 8, a real current flowing from the end surface of the dielectric material 4 into the conductive case 2 diverges from the center of the end surface of the case radially toward the peripheral surface of the case and the current flows on the peripheral surface of the case in parallel with the center axis of the dielectric cylinder 4 into the central portion of the other end surface of the case 2.
However, as shown in FIG. 9, the conventional conductive case 2 comprises a case upper lid 201, a case side portion 202 and a case lower lid 203 in combination. Therefore, interfaces 204 and 205 are formed, as shown in FIG. 1, in the conductive case 2 at a contact portion between the upper lid 201 and the side portion 202 and a contact portion between the lower lid 203 and the side portion 202. These interfaces 204 and 205 are formed in the direction perpendicular to the direction of a flow of a real current. However, by forming an interface in the conductor in the direction perpendicular to the direction of the flow of the real current i.e. in the direction intersecting the direction of a flow of the current, the resistance at that portion is increased, resulting in a loss of power P, represented as P=I.sup.2 R. As a result, a joule heat is generated at the interface contact portion and the no-load quality factor Q is decreased. An approach for solving this problem is set forth in the following.
FIG. 10 is a perspective view of a conductor case, in which the conductor case 25 is shown as disassembled. As shown in FIG. 10, the conductor case 25 comprises symmetrical case portions 25a and 25b separable in the plane including the center axis 401 of a cylindrical dielectric member 4 disposed in the center of the case 25. The case portions 25a and 25b as combined are fixed with fixing screws. As a result, a joining surface 26 of the case 25 is formed in parallel with the center axis 401 of the dielectric material 4. In other words, the joining surface 26 is formed in parallel with the direction of a flow of a real current (FIG. 8) flowing in the above described case which is not the direction intersecting the direction of a flow of a real current. As a result, no contact resistance is interposed on the joining surface 26 against a flow of a real current and accordingly little current loss is caused and the no-load quality factor is not decreased.
FIGS. 11A and 11B are views showing other manners of dividing the conductive case. As shown in FIG. 11A, insofar as the case 25 is divided into a plane or planes including the center axis of the dielectric material, the case 25 may be not only a combination of two separated portions but also a combination of four separated portions. As shown in FIG. 11B, the case 25 may be a combination of three separated case portions or may be any other combination of otherwise separated case portions.
Meanwhile, since the above described dielectric resonators shown in FIGS. 1 to 11B employ a metallic conductive case, the same unavoidably become expensive. The reason is that the necessity of improving the temperature characteristic of the resonance frequency in the light of a difference between the coefficients of linear expansion of the metal and dielectric material complicates the structure of the conductive case, as shown in FIGS. 3 to 7. It also increases the number of components and the number of working steps, resulting in less suitability for mass production.